CJT wrote:
Robert Allison wrote:
snip

Let me rephrase the equality issue. If you don't have an equal amount
of ridge and soffit vents, then your ventilation will not be perfect.
If you have a difference, it is best to have more ridge or passive
venting. But if you don't have an equal amount, it will not be worse,
it will just not be as good as it could be.


I don't see the issue. Having unequal areas just means the velocities
will differ. You will never have soffits distributed over the entire
relevant area, nor is that necessary.

Yes, I know that. The OP had concerns about it, so I was
trying to explain why perfection is impossible and why it is
not a disaster if perfection is not achieved.

The one drawback to having too few soffit vents (less sf than ridge
vents) is that the attic may tend to draw air out of the house itself,
thus sucking out conditioned air.


I don't see how that follows. How about some physics?

OK, first take the case of ridge vents with NO soffit venting.
The air in the attic heats up, hot air rises and passes out
of the ridge vent. Now there is negative pressure in the
attic. Air needs to come from somewhere. Generally, it will
come back in through the ridge vent, but the negative air
pressure in the attic tries to draw air from wherever it can.
So if you have ceiling light fixture with a bad cut around
the box and it has not been sealed with foam, then air will
pass through the openings drawing conditioned air from the
house. Same with any opening that can go to the attic space.

It is the same with less soffit vent sfootage than ridge vent
sfootage, only less so. So you want more soffit vents than
ridge vents to lessen this effect. It never entirely goes
away, but is negligible with more vents on the soffit side.

(This happens around ceiling lights,

ac vents or other ceiling penetrations and even through outlets if the
top plates is not foamed.)

So, if you have to have an imbalance, it is best to have more soffit
venting than ridge. The air will still move out through the ridge
vents, causing negative pressure in the attic, thus drawing fresh air
in through the soffit.





--
Robert Allison
Rimshot, Inc.
Georgetown, TX